Hearing aids are portable hearing apparatuses for use by the hard of hearing. In order to meet the numerous individual requirements, different hearing aids types are available, such as behind-the-ear (BTE) hearing aids, in-the-ear (ITE) hearing aids, and concha hearing aids. The hearing devices listed by way of example are worn in the outer ear or in the auditory canal. However, bone conduction hearing aids, implantable or vibrotactile hearing aids are also commercially available. In these cases, the damaged hearing is stimulated either mechanically or electrically.
The basic components of a hearing aid are essentially an input transducer, an amplifier and an output transducer. The input transducer is generally a sound receiver, e.g. a microphone, and/or an electromagnetic receiver such as an induction coil. The output transducer is mainly implemented as an electroacoustic transducer, e.g. a miniature loudspeaker, or as an electromechanical transducer such as a bone conduction earpiece. The amplifier is usually incorporated in a signal processing unit. This basic design is shown in FIG. 1 using the example of a behind-the-ear hearing aid. Installed in a hearing aid housing 1 for wearing behind-the-ear are one or more microphones 2 for picking up sound from the environment. A signal processing unit 3 which is likewise incorporated in the hearing aid housing 1 processes the microphone signals and amplifies them. The output signal of the signal processing unit 3 is transmitted to a loudspeaker or earpiece 4 which outputs an audible signal. The sound is in some cases transmitted to the wearer's eardrum via a sound tube which is fixed in the auditory canal using an earmold. The hearing aid and in particular the signal processing unit 3 are powered by a battery 5 likewise incorporated in the hearing aid housing 1.
Monaural noise reduction methods are a fixed component of hearing aids. Frequency-domain methods using spectral weighting, e.g. Wiener filter or spectral subtraction, are used for this purpose.
With these noise reduction methods, the noise component must be estimated from the received noisy signal. For this estimation, the minimum statistics method, for example, can be used. In addition to noise estimation, estimation of the amplitude spectrum of the wanted signal is also necessary for Ephraim-Malah spectral weighting.
Both the wanted signal estimation algorithms and the noise signal estimation algorithms are based on particular, mainly simplifying assumptions in respect of the signal statistic. Thus, for example, for determining the Ephraim-Malah weighting rules, the wanted signal amplitude spectra are assumed to be Gauss distributed (cf. EPHRAIM, Y.; MALAH, D.: Speech Enhancement Using a Minimum Mean-Square Error Short-Time Spectral Amplitude Estimator. In: IEEE Transactions on Acoustics, Speech and Signal Processing, December 1984, Vol. ASSP-32 No. 6, pages 1109-1121).
However, the actual statistical characteristics of the wanted and noise signal are usually much more complex and are therefore only taken into consideration to a limited extent in the methods mentioned. In addition, the parameters for optimum noise reduction effect with minimal wanted signal distortion are normally in fixed settings during operation.
In the case of non-static noise, the effect of the noise reduction methods mentioned is severely limited. Because the signal statistic has to be acquired over a sufficiently long time, the estimation of a high time-domain dynamic range of the noise can follow only relatively slowly. This reduces the noise reduction effect in such situations.
With the hitherto known methods, no a priori available information is utilized for acquiring the signal statistic. Even if the signal statistic is taken into account, in all the methods only a finite number of statistically different signal models are used during operation. These signal models are all in a fixed form. In addition, the modeling, particularly for the wanted signal, is very complex and also mainly defined for one type of signal such as voice. The noise modeling is also mainly limited to the spectral envelope only. This means that in the case generally arising in practice, a plurality of spatially separated noise signals can be mapped only with great difficulty. Both the spatial and the spectral characteristics may also change over time.
Publication DE 101 14 101 A1 discloses a method for processing an input signal in a signal processing unit of a hearing aid. In the hearing aid, adjustment parameters of a signal processing unit which relate to noise reduction are set as a function of the result of signal analysis of the input signal. If noise signals are detected, they are assigned to different noise signal categories. Different noise reduction algorithms are activated and deactivated depending on the noise signal category determined.